WO95/10160 discloses a video display apparatus which receives a transmitted color video signal being coded using three system primaries, and which decodes the signal for display on a device using four display primaries. The four display primaries are independent in that no display primary can be expressed as a combination of another two of the display primaries, and thus define a quadrilateral. A fifth, imaginary display primary is determined which is a linear combination of the third and the fourth display primaries. Now, the quadrilateral is divided into three triads defined by respectively the first and fourth and fifth, the first and fifth and second, and the second and fifth and third display primaries. The received video signal is decoded by three matrix arithmetic units, one for each triad, to calculate drive signals for the first to fourth display primaries. For each pixel, the matrix arithmetic unit output producing no negative display drive signal is selected to drive the four-primary display device. Thus, for a particular input pixel of the input signal the triad is selected which enables to display the input pixel color.
In an embodiment of this prior art, the following steps are performed. Firstly, the fifth display primary is determined as a linear combination of the third and fourth display primaries. Secondly, the signals for generating the first, second and fifth display primaries are calculated using a triad of display primaries comprising the first, second and fifth display primaries. Thirdly, if the signals calculated are all either zero or positive, the signals for the first and the second display primaries are used to form display primary drive signals, and the drive signals for the third and the fourth display primaries are calculated by using the signal for the fifth display primary to calculate drive signals for the third and the fourth display primaries according to the linear relationship between them. Fourthly, if the signal calculated for the first display primary is negative, display primary drive signals are initially calculated is if they are all either zero or positive, and then the second and fourth display primary drive signals calculated are modified by the addition of predetermined multiples of the first display primary drive signal thereto and the first display primary signal is set to zero. A similar approach is followed if the signal calculated for the second display primary is negative.
In the prior art, the fifth primary is introduced to be able to generate the triads. The computations in each one of the triads are simple because in each triad the three primary input signal is deterministically converted into a three primary display signal. However, the prior art approach has the drawback that discontinuities may occur across borders between different triads. If in the now following the term multi-primary is used, it indicates that more than three display primaries are present.